Closed-loop monitoring and identification of CD alignment for papermaking processes

Abstract

Alignment is a critical component for modeling a cross-directional (CD) papermaking process. It specifies the spatial relationship between individual CD actuators to paper quality measurements. Misalignment can occur unexpectedly due to sheet wander or CD shrinkage variation. In certain applications and circumstances, a misalignment of one third (⅓) actuator zone width can result in significant paper quality degradation. Detecting a misalignment and identifying CD alignment in closed loop are highly demanded in paper mills but these are nontrivial problems. A technique for maintaining proper CD alignment in sheetmaking systems entails monitoring the alignment online, triggering closed loop identification if misalignment is detected, and then deploying the new alignment. No personnel intervention is required.

Description

FIELD OF THE INVENTION[0001]The present invention generally relates to techniques for monitoring and controlling continuous sheetmaking systems such as a papermaking machine and more, specifically to maintaining proper cross-directional (CD) alignment in sheetmaking systems by monitoring control performance in real time, detecting a misalignment, identifying the alignment in closed-loop, and updating a CD controller with the correct alignment model.BACKGROUND OF THE INVENTION[0002]In the art of making paper with modern high-speed machines, sheet properties must be continually monitored and controlled to assure sheet quality and to minimize the amount of finished product that is rejected when there is an upset in the manufacturing process. The sheet variables that are most often measured include basis weight, moisture content, gloss, and caliper (i.e., thickness) of the sheets at various stages in the manufacturing process. These process variables are typically controlled by, for exa...

AI Technical Summary

Benefits of technology

[0009]To detect misalignment, the inventive method measures “actuator picketing,” which refers to a specific actuator setpoint profile pattern that is dominated by high spatial frequency components and looks similar to a picket fence. This phenomenon is a well-known symptom associated with CD alignment problems. For a well-tuned and well-aligned CD controller, the actuator setpoint profile typically contains a limited amount of high, spatial frequency components. After performing spectrum analysis on actuator setpoint profile, if the accumulated power within a certain high spatial frequency band exceeds a pre-specified threshold, one can conclude that the actuator picketing is detected and the misalignment is present. The pre-specified threshold is defined by carrying on a controller performance baselining, which is an effective way to quantify control performance and determine the thresholds for picketing detection. To improve the detection algorithm robustness, the spectrum analysis for measurement profiles can be optionally added in the online monitoring of present invention. This invention is able to avoid the fault detection caused by overly aggressive controller tuning after adding measurement profiles into the analysis. The misalignment detection method of the present invention can account for the effects of spatial response shape change that is needed for predicting the outputs accurately.

[0010]With respect to alignment identification, the present invention employs an alignment identification algorithm that is able to extract the open-loop shape response using closed-loop experimental data. The algorithm can tolerate 100% process time-delay uncertainties and, in addition, CD alignment is identified by one-step optimization instead of iterative updating. A novel closed loop intelligent PRBS (Pseudo-Random Binary Sequence) test is introduced in the closed-loop identification. The magnitude, location and duration of PRBS excitation can be automatically determined by this invention based on the constraints and setpoints of CD actuators. Compared with traditional persistent “bump,” PRBS tests reduce the additional CD variances in the sheet triggered by identification experiments. Because of the nature of closed-loop tests, process disturbances can still be rejected by feedback controllers during the identification. A matrix inversion formula is employed to extract the open loop responses from closed-loop experiment data. Statistic signal processing and constrained nonlinear optimization techniques are adopted for full response shape identification. Although this algorithm is particularly suited for alignment identification, it can be extended to identify the entire CD spatial model in closed loop. Both the linear and nonlinear shrinkage are supported by the present invention.

Problems solved by technology

This phenomenon is a well-known symptom associated with CD alignment problems.

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